Thousands of Syrians have poured into the streets calling for the downfall of the president, Bashar al-Assad, and expressing support for army defectors fighting the regime, activists say.

The protests came as the UN's top human rights official urged the international community to take "immediate measures" to protect civilians in Syria.

The country's protest movement gave its most explicit show of support so far to army defectors who have reportedly clashed with loyalists in northern and central Syria in an increasing militarisation of the seven-month uprising.

The Syria-based activist Mustafa Osso and the Local Co-ordination Committees, an activist group, said the protests had spread from the suburbs of the capital, Damascus, to the southern province of Deraa, the northern provinces of Aleppo, Idlib and Hassakeh, and the central regions of Homs and Hama, as well as to other areas.

The opposition had called for protests after Friday prayers in support of the "free officers", in reference to army defectors who have been fighting regime troops over the past weeks. Clashes between troops and gunmen believed to be defectors left at least 25 people dead on Thursday, according to the London-based Syrian Observatory for Human Rights.

The uprising against Assad's regime began in mid-March amid a wave of anti-government protests in the Arab world that toppled autocrats in Tunisia, Egypt and Libya. Assad has responded with a fierce crackdown.

In Geneva, Navi Pillay, the UN high commissioner for human rights, warned that the unrelenting crackdown by the government could worsen unless further action is taken. She said the death toll from seven months of anti-government unrest in the country had risen above 3,000.

"The onus is on all members of the international community to take protective action in a collective and decisive manner, before the continual ruthless repression and killings drive the country into a full-blown civil war," Pillay said in a statement.

She didn't elaborate on what measures the international community could take beyond the sanctions already imposed on Assad's regime. Her spokesman, Rupert Colville, told reporters in Geneva that it was up to the UN security council to decide what action was appropriate.

But he added: "What has been done so far is not producing results and people continue to be killed every single day. "Just hoping things will get better isn't good enough, clearly."

The UN human rights office estimates that more than 3,000 people have been killed since mid-March – about 10 to 15 people every day. The figure includes at least 187 children. More than 100 people had been killed in the past 10 days alone, it said.

Colville said hundreds more protesters had been arrested, detained, tortured and disappeared. Families of anti-government protesters inside and outside the country have also been targeted for harassment.